Uncoupling yeast growth and product formation in chemostat and retentostat cultures

The progress of modern biotechnology has enabled the development of fermentation processes for the production of fuels and chemicals from renewable feedstocks. The current fermentation processes for bio-based production commonly start with a growth phase of the microorganisms followed by a production phase. This implies that biomass formation competes with the production of the desired product in terms of consumption of the feedstock. In industrial fermentations, maximizing the product yield, in other words, minimizing the substrate flux to biomass, CO2 and byproducts is the primary goal. To reach this objective, the uncoupling of microbial growth from product formation seems like a feasible approach, providing that the microbial host maintains high productivity in the absence of growth. The research presented in this thesis aims to improve understanding of the physiology of microbial at near-zero growth rates and thereby provide insights for the design of industrial fermentation processes based on the zero-growth concept. Specifically, S. cerevisiae was applied as the microbial cell factory, and succinic acid, a non-catabolic product and its synthesis from sugar requires a net input of ATP, was chosen as a model product. The microbe was cultivated in the chemostat and retentostat mode under industrially relevant conditions, as reflected by aerobic cultivation at low pH and at a high dissolved CO2 level. To the end, the quantitative physiology of yeast at slow and near-zero growth states was investigated, and uncoupling S. cerevisiae growth and succinic acid production was achieved with considerable succinic acid productivity. This study illustrates the potential for high-yield production of non-dissimilatory products at near-zero growth rates, with growth being limited by nutrients other than the carbon and energy source. In addition, it highlights a requirement for further research into enhancing strain robustness under industrial conditions, with specific attention for low-pH tolerance.